The pressure switch is looking at the up stream hyd pressure to the thrust reverser. on the ground pressure would be expected at that point and via the air ground logic the light would be off.in the air the pressure switch should not see pressure if it does via the air ground logic the light would be on telling the crew that there was hyd pressure at the thrust reverser an unsafe situation with the possibility that if the flight crew were foolish enough to try and deploy the thrust rev in flight it may deploy. The schematic does not give the full story only the mechanical bits!!